1. Field of the Invention
The present invention generally relates to a vibration wave driving apparatus, and more particularly, to a configuration of a vibration element used in a bar-shaped vibration wave driving apparatus.
2. Related Background Art
A bar-shaped vibration wave driving apparatus includes, as a basic structure, a vibration element composed of elastic members made of metal or the like and a piezoelectric element as an electro-mechanical energy conversion element. The bar-shaped vibration wave driving apparatus generates a driving vibration such as a traveling wave or the like through application of an alternating voltage as an alternating signal with different phases to the piezoelectric element.
A contact member is brought into pressure contact with a driving part of the elastic member by a pressurizing means and the contact member is frictionally driven by the driving vibration generated in the driving part of the elastic member to allow the vibration element and the contact member to be moved relative to each other.
There is a vibration wave motor as an example of such a vibration wave driving apparatus in which a vibration element is used as a stator and a contact member as a rotor.
Examples of the vibration element of the vibration wave motor include those with a configuration in which a ring-shaped piezoelectric element plate is attached to one surface of a ring- or disc-shaped elastic member and those of a type in which the rotation of the rotor is taken out through an output shaft or of a type in which the rotation of the rotor is taken out directly.
Such a vibration wave motor has been applied to products to be used for driving a camera lens and the like. There are annular type and bar-shaped type vibration wave motors.
FIG. 11A is a structural view of a bar-shaped vibration element of a bar-shaped vibration wave motor used for driving a camera lens. FIG. 11B shows a vibration mode (with the z-axis assigned to the axial direction and the r-axis assigned to the radial direction) in an axis part of the bar-shaped vibration element.
Numeral 101 indicates a first elastic member; numeral 102, a second elastic member; and numeral 103, a piezoelectric element. Numeral 106 denotes a shaft member passing through the first elastic member 101, the piezoelectric element 103, and the second elastic member 102. One end of the shaft member 106 located on the side of a rotor 108 is fixed to a fitting member 107 to be attached to a product and the other end is fixed to a nut 115. A threaded portion is formed in the other end of the shaft member 106. With the nut 115 tightened, the first elastic member 101, the piezoelectric element 103, and the second elastic member 102 disposed between a flange portion provided for the shaft member 106 and the nut 115 are sandwiched and fixed therebetween. Numeral 108 indicates the rotor as described above, and numeral 116 denotes a friction member fixed to the first elastic element 101 to be in contact with the rotor.
When a driving signal is applied to the piezoelectric element 103, the bending vibration indicated in FIG. 11B is excited in the bar-shaped vibration element and thereby the bar-shaped vibration element makes a swing movement substantially about the z-axis. Accordingly, the friction member 116 makes a circular motion around the z-axis.
The vibration element of such a bar-shaped vibration wave driving apparatus has been reduced in size in its radial direction, but there is still room for reduction in size in its thrust direction, i.e., in the length of its axis.
However, when the vibration element is simply shortened, there arise problems that the resonance frequency increases and the vibration displacement is reduced, which causes the deterioration in efficiency of friction drive, the increase in price of a driving circuit element due to the high frequency, or the increase in loss inside the element.
Hence, Japanese Patent Application Laid-open No. 4-91668 proposes one in which a vibration element is provided with a smaller diameter portion to reduce the resonance frequency. According to this proposal, however, when the vibration element is simply made thinner to lower the resonance frequency, the diameters of a piezoelectric element and a frictional surface also are reduced and thereby generating force of the piezoelectric element and the friction torque also decrease.
As a technique for shortening the axis of a bar-shaped vibration wave driving apparatus, for solving such matter, there is one disclosed in Japanese Patent Application Laid-open No. 2001-145376, which is shown in FIG. 12.
A vibration apparatus in such document is identical to a conventional product in that a piezoelectric element 203 is sandwiched and fixed between a first elastic member 201 and a second elastic member 202. However, the apparatus is different from the conventional product in that the first elastic member 201 with a frictional surface is divided into two regions including an inner peripheral portion and an outer peripheral portion that are connected to each other through a thin connection part 210.
According to this structure, even if the axis of the bar-shaped elastic member is shortened, a low resonance frequency can be obtained since the first elastic member has a sufficiently high mass.
According to this technique, however, when the connection part 210 is made thinner to allow the resonance frequency to be lowered and thereby its stiffness is deteriorated, the displacement generated in the piezoelectric element is absorbed by a soft spring of the connection part 210. Consequently, it is difficult to transmit the driving force to a rotor efficiently. On the contrary, when the connection part 210 is made thick, the resonance frequency cannot be reduced effectively. Thus, it seems that there is still room for further improvement.